User Equipment (UE or UEs in plural) in a cellular network, such as Third Generation Partnership Project (3GPP) Universal Mobile Telecommunications System (UMTS) or Long Term Evolution (LTE), make use of a random access procedure to gain access to the network. According to this procedure, UEs randomly choose a signature (a temporary identifier) from a group of broadcast signatures and attempt to access the network in some predefined time slots when it has data to send, for signalling purposes or both.
In such scenarios, there is a risk that two UEs make use of the same signature at the same time and a collision results. The network operator can dimension the network with an adequate number of signatures and with sufficient access opportunities to keep the collision probability low (typically 1%).
Machine Type Communication (MTC) devices (sometimes referred to as Machine-to-Machine or M2M devices) are increasingly being used for a variety of applications. Their numbers are expected to grow at a high rate. MTC devices are typically automated data reporting systems such as utility meters or status reporting devices. Since the volume of data transmitted and received by such devices is low, cellular networks may not expect the large numbers of MTC devices to require significant changes to network dimensions.
Such devices may be programmed to send data at a specific time, often at a time when network activity is normally low (for example, late at night). It is therefore likely that a large number of MTC devices may be configured to send data at or around the same time. If a large number of MTC devices try to send respective access messages over the Random Access Channel (RACH) of the network at the same time, the resources reserved by the network for random access would be overwhelmed.
There are existing approaches to deal with random access messages from different UEs colliding (that is being transmitted simultaneously). In one approach, each UE is expected to wait (known as backing-off) for a time interval of random length, before attempting a further random access message. However, the more collisions that occur, the longer will be the time taken by UE to access the network. This will result in a longer call setup delay, which is detrimental to the user experience.
If a large number of MTC devices try to access the network simultaneously, the number of collisions will significantly increase. In view of the large number of devices, simply backing-off for a random interval before trying again will not solve the problem. Moreover, the user experience for other UEs will become extremely degraded.
Another approach for dealing with large RACH intensity is known as Access Class Barring. This was designed to cope with very rare situations, where a large number of UEs all try to access the network at the same time. For instance, it is well known that on special occasions (such as New Year's Eve), during a sporting event or in an emergency situation, the network may experience a surge in network access attempts over the RACH by UEs.
Access Class Barring is available in UMTS and LTE networks to deal with this situation. As explained in 3GPP TS 22.011, which is incorporated herein by reference, all UEs are members of one out of ten randomly allocated mobile populations, defined as Access Classes 0 to 9. The Access Class is stored in the Subscriber Identity Module (SIM) or Universal Subscriber Identity Module (USIM) of the UE. In addition, UEs may be members of one or more out of 5 special categories (Access Classes 11 to 15), also held in the SIM or USIM, which are allocated to specific high priority users. In LTE for example, the network can prevent a certain fraction of UEs from accessing the network for a certain time. This is achieved by broadcasting information about the barring factor to use, the time UEs should be barred from accessing the network and to which type of traffic the barring applies, for instance Mobile Originating (MO)-signalling, MO-Data. Since, the allocation of Access Classes is random, by barring a specific class or classes, the barred UEs are randomly selected. Moreover, in order to allow emergency services and operator to still have access to the network in congestion situations, special access classes are used by those UEs such that they are not barred from access.
The main drawback of this mechanism is that it is intended for use during certain rarely occurring events. Consequently, the number of Access Classes is kept small. It is not practical to increase the number of Access Classes, for example to provide a special access class for MTC devices, as this would require a significant adjustment to the UE SIM and significantly increased communications overheads. Also, Access Class Barring is currently designed to prevent access to the network completely, whereas it is desirable that MTC devices access network regularly.
A third approach is to program the MTC devices to send their data at different times. However, considering the large number of devices and the lack of time synchronisation among the devices, the burden of such a solution would be on the end user. It is impractical to expect the end user to take any measures to protect the cellular network. Instead, it is desirable for the network to adapt itself to the devices. In view of the foregoing, this presents a significant challenge, if it is to be achieved without adding significant further communication overheads to the data traffic between the network and UEs.